You're a pantser if you use seat of the pants story telling, or writing without benefit of an outline. The real question is whether this is a good trait, or whether you should immediately abandon the practice.
There are two schools of thought on the issue. On one side you have Chris Baty's No Plot, No Problem. This philosophy drives the concept of Nanowrimo with it's thousands of writers working to finish a novel each November. Stephen King in On Writing suggests something similar. If you have a great idea, just start writing and see where it takes you. One could argue that much of what happens in Nanowrimo, while good for the individual isn't publishable. However, no once can argue that King isn't a great story teller.
The other side of the argument states that you're basically wasting your time if you don't structure your story and use at least a rudimentary outline. Larry Brooks in Story Engineering states that you need to structure your story in a series of plot points, basically conforming to the quarters of the story, so that you continually draw the reader forward. This is essentially the format used in screen writing.
So what's right. Recognizing that few of us can compete with Stephen King, should we outline before we write? Personally, I think a writer should use whatever method makes him or her comfortable. I can see drawbacks with either mode. If you're a pantser, you may end up rewriting to make your story conform to a plot line. On the other hand, if you plot too tight, you may miss character interactions that would make your story special.
So what's the advice? If you have a good feel for story arc (and Stephen King is apparently one of those writers.) you may do your best work by having the idea and letting your characters tell the story. If you find yourself muddled about a third of the way through the story, not sure of where to go, I'd suggest outlining and trying to fit your story to major plot points.